1. Field of the Invention
This invention relates to a heat sensitive flow sensor for detecting the flow rate of a fluid such as a gas or liquid and, more specifically, to a heat sensitive flow sensor suitable for detecting the amount of air flowing into an internal combustion engine of an automobile, for example.
2. Description of the Prior Art
FIG. 24 is a partial sectional view of a conventional heat sensitive flow sensor as shown in Laid-open Japanese Patent Application No. Hei 5-1930, for example. In the figure, reference numeral 1 denotes a main conduit in which air passing through an air cleaner and circulates, and 2 a flow straightener, composed of a honeycomb face plate, for straightening an intake air flow and correcting a flow velocity distribution or the like. Reference numeral 3 represents a metering pipe whose air inflow side is shaped like a reversed cone and which is fitted with a heat resistor 4 for metering the volume of intake air, 5 a strut for supporting the metering pipe, 6 a control circuit portion, and 7 a temperature compensation resistor for metering the temperature of intake air.
This heat sensitive flow sensor is installed in an air introduction passage of an internal combustion engine to output a signal corresponding to the flow rate of intake air. The output of the flow rate of intake air is sent to an unshown engine control circuit as a factor of controlling an engine.
That is, air passing through the air cleaner of the internal combustion engine flows through the flow straightener 2 and part of the air flows into the metering pipe 3. At this point, the quantity of heat corresponding to the volume of intake air is absorbed from the heat resistor 4. However, since the circuit portion 6 controls such that the difference between the temperature of the part of air flowing into the metering pipe 3 and the temperature of intake air measured by the temperature compensation resistor 7 should be kept almost constant regardless of the volume of intake air, the volume of intake air can be measured from the value of a current running through the heat resistor 4.
Efforts are being made to reduce the sizes of and integrate devices and parts installed in an engine room. For instance, in an air intake system for a fuel injection system, a heat sensitive flow sensor is arranged in the vicinity of a throttle valve or an air intake duct to be installed at an upstream or downstream of the sensor is curved to reduce the length of an air intake passage in some cases.
When the throttle valve is installed in the vicinity of a downstream side of the heat sensitive flow sensor as described above and the opening of the throttle valve is small, a drift in the main conduit, that is, an air flow leaning to an upper; lower, right or left portion of the metering pipe 3 becomes large, and a flow in the metering pipe is affected by that flow. Therefore, when the opening of the throttle valve is small, there is such a problem that the metering accuracy of the volume of intake air lowers. When a curved air intake duct is arranged on an upstream side of the heat sensitive flow sensor, the above drift is also produced in the air intake passage. This drift is greatly affected by a stain on an air cleaner element installed at a further upstream of the heat sensitive flow sensor, whereby a flow in the metering pipe is changed with the result that the metering accuracy of the volume of intake air lowers along with the passage of time. The degree of a reduction in the metering accuracy is relatively larger than when the air intake duct arranged at an upstream of the heat sensitive flow sensor is not curved.
The present invention has been made to solve the above problems and it is therefore an object of the present invention to provide a heat sensitive flow sensor which is capable of stabilizing an inflow and outflow in a metering pipe by suppressing a drift in a main conduit and reducing a metering error of the volume of intake air even when a drift source such as a throttle valve is present in the vicinity of a downstream side of the heat sensitive flow sensor or when an air intake duct to be installed at an upstream and downstream of the sensor is curved.
It is another object of the present invention to reduce a pressure loss which is generated at an upstream and downstream of the heat sensitive flow sensor.